This project intends to study the effects of micrometeorologic conditions on the development of the eggs and larvae of the gastrointestinal nematodes of sheep, Ostertagic circumcincta, Cooperia spp., Bunostomum trigonocephalum, Trichostrongylus axei, etc., and also of the survival of these stages on pasture. Maximum, minimum and mean temperatures will be measured at the soil surface and in a standard weather shelter, precipitation, soil moisture, relative humidity, snowfall and snow cover, and other micrometeorologic factors will be measured, and these will be correlated by means of the IBM 360 computer with the development and survival of the free-living nematode stages. The effects of certain of these factors will also be studied under controlled conditions in the laboratory, and the roles of possible invertebrate transport hosts (if any) will be determined. Comparisons will be made with information already gained in similar studies on Haemonchus contortus and Trichostrongylus colubriformis. This work will be used as a model for epidemiologic studies of other diseases and disease agents of man and other animals.